Pest Resistance in Elms

Just about everyone is familiar with the devastating effect Dutch elm disease (DED) has had on populations of American elm. Much research has gone into developing trees resistant to this disease. A number of resistant elms have been identified, some of them natives, more from East Asia. Unfortunately, Dutch elm disease isn't the only problem to afflict elms. A number of other pests can cause problems on these trees, including Japanese beetles, gall aphids and leafminers. One of the latter, the European elm flea weevil, is a leaf miner that was first found in this country in 2003 and is expected to spread across the US.

The National Elm Trial (NET) is a program in which elms are being evaluated in 15 states for resistance to DED as well as other pests. As reported in the July-August 2010 issue of Hortideas, evaluations at the NET site in Lexington, Kentucky by researchers at the University of Kentucky Department of Entomology have shown that all of the DED-resistant elms in the trial were susceptible to at least some of these other pests. American elm cultivars such as 'Valley Forge', 'Princeton' and 'New Harmony' were more susceptible to the leafminer Agromyza aristata than hybrid elms. But some of the hybrids, such as 'New Horizon' were more susceptible to sawfly and gall aphid damage. Most of the hybrid elms were susceptible to European flea weevil infestation; 'Morton Accolade' was the least susceptible hybrid tested. 'Jefferson' American elm was quite resistant to this flea weevil; it was, however, a favorite of Japanese beetles.

Research continues under the NET program, its goal being to find elms that not only resist DED, but that resist other troublesome pests as well, in the hopes that one day these stately trees can again grace our landscapes.